Numerous operations are performed during the drilling and maintenance of subterranean wells that require the introduction of various fluids into the well for specific purposes. For example, fluids may be introduced into the well for the performance of gravel packing operations, sand treatment operations, or other completion or service operations. Such fluids as acids, cements, polymers, and sand-filled liquids may be injected into the formation or into the outer annulus between a sand screen and a perforated well casing. After the various operations are performed, completion fluids are introduced into the well to displace the service fluids that were used to perform the various operations.
Once the completion fluid introduction operation is complete, the apparatus used for the operation must be removed along with the tubular work string carrying the apparatus. As the apparatus is removed, however, quantities of completion fluid contained within the apparatus and work string may be lost. For example, the completion fluid may be spilt into the formation as the apparatus and work string is removed. The loss of completion fluid is undesirable since completion fluid is costly and will contaminate the formation if it is not contained.
Several methods have been developed for preventing completion fluid from being spilt into the formation. Those methods include introducing viscous pills, loss circulating material and/or gel material in the bore as the work string is withdrawn in order to protect the formation from the completion fluid. Such materials may be used to seal leak paths.
Still another method used for containing completion fluids is that of an automatically operating flapper valve. Such valves have been conventionally mounted on a screen support sub between the screen and a packer for pivotal movement from an upright, open bore position, to a horizontal, closed bore position. The flapper valve is propped open in the upright position during the various completion and service operations. When the work string and the apparatus are pulled out, the flapper valve is moved into the horizontal position against a valve seat, usually by a biasing mechanism. The closed valve keeps the completion fluid contained above the valve until another tubing string is inserted into the well.
Conventional flapper valves are generally not compatible, however, with expandable tubing, which is of a reduced diameter during installation and is expanded to an increased diameter after the tubing is in place within the borehole. In its unexpanded state, expandable tubing facilitates installation in offset, slanted, or horizontal boreholes. Upon expansion, solid or perforated tubing and screens provide support for uncased borehole walls while screening and filtering out sand and other produced solid materials which can damage the tubing. After expansion, the internal diameter of the tubing is increased, thereby improving the flow of fluids through the tubing. Because a flapper valve is typically not moved into the horizontal, or closed, position until after the tubing is expanded to the increased diameter, however, the flapper valve may not form a sufficient seal with the valve seat. As a result, a flapper valve incorporated into expandable tubing may not be effective to inhibit the loss of completion fluid.